Supple sheets are already known and used for packaging, for example, surgical equipment, probes, scalpels, clamps, and any other equipment used by doctors and/or surgeons.
Such supple sheets are required to have, simultaneously and necessarily, definite properties.
The first property is that the sheet must be supple in order to form bags designed to receive objects of various sizes and shapes.
The second property is that the sheet must be resistant to tearing as well as to bursting, as indeed, it is designed to contain objects which may be heavy and blunt or sharp. It should also be able to withstand, as a bag, during sterilization with gases such as ethylene oxide, a pressure when the gases are injected, and subsequently a strong depression during exhaust of the gases which must be total.
The third property is that the sheet must have a low power of absorption towards water, so that the water or any other liquid products cannot penetrate inwardly from the outside through the bag formed by the sheet, and soil the object contained therein.
The fourth property is that the sheet must be permeable to gases. Indeed, when the object has been introduced into the bag and said bag has been sealed, the whole is subjected to the action of sterilizing gases and/or of sterilizing ionic radiations. However, it is important that such permeability be low enough to prevent bacteria or other micro-organisms from penetrating into the bag.
It is already known, for example for forming packages for surgical tools, to use sheets which are constituted of papers obtained by the wet process, by draining an aqueous suspension of cellulose fibers on a wire (Foudrinier process) and drying the sheet.
Such paper sheets are sold to manufacturers of sterilizable bags or packages who form the bag by depositing a bonding layer of varnish between the edges of the sheet and a transparent film.
Other converting techniques use a heat-sealable lacquer in aqueous or solvent medium, permitting to the paper to retain a certain gas-permeability. The bags or pouches are then produced by heat sealing with plastic films.
One drawback with these papers is that, generally, they lack suppleness and are moderately resistant to tearing and bursting.
Said papers have a low water- or other liquids-absorbing power, due to the introduction, at the production stage, either in the aqueous suspension, or by surface treatment, of a bonding agent, which may be neutral, for example of dimeric ketene type which reduces water absorption through the sheet.
The aim of the prior art has therefore been to improve the physical strength of the sheet, and more particularly its resistance to tearing and bursting. To this effect, attempts have been made to increase the bond between the cellulosic fibers. To do this, a binding agent of polyvinyl alcohol type has been introduced by coating the sheet with a composition containing said polyvinyl alcohol. However, one disadvantage with this mechanically resistant paper for sterilizing is that the introduction of the binder gives it too much stiffness, resulting in an unaesthetic appearance of the final package caused by the presence of creases which are due to the fact that the package does not adequately adopt the shape of the packed objects.
Other sheets of paper are known which are made more resistant to tearing and bursting by mass and/or surface introduction of, for example acrylic latex of polymers. The lack of suppleness of the sheet of paper may cause opening of the seals during handling of the finished packages. Said sheets are produced and sold by the company KIMBERLEY CLARK. They are supple and have a plastic feel. They are, however, difficult and expensive to produce and give a paper of non-homogeneous properties. Furthermore, the paper is relatively permanent and much less biodegradable than a normal paper. Said papers are not very water-repellent as they are of low sizing.
Other known sheets of paper are treated with sorbitol in a size-press to make them supple. The problem arising with such sheets is that their resistance to tearing and to bursting is relatively poor and that sizing is difficult to obtain.
Non-woven dry type sheets have also been produced, the method consisting in producing a mat of synthetic fibers, such as polyethylene fibers, by thermobonding the fibers together. Said sheets are produced by the company DU PONT DE NEMOURS and sold under the name TYVEK.RTM.. They are resistant to tearing and to bursting. Moreover, they are supple. They have nevertheless a major drawback which is that their look-through is very heterogeneous, i.e. that the distribution of fibers per surface unit is irregular, consequently that the porosity of the sheet is uneven. For example, in some parts, the sheet may have pores of very large diameter which will allow the micro-organisms inside the package produced with the sheet. In other parts, on the contrary, the sheet may have pores of extremely small diameter, which will hinder the penetration of the molecules of sterilizing gas. Moreover, said sheets are absolutely permanent, i.e. non biodegradable.